1. Field of the Invention
The present invention relates to an apparatus for reading an optical disk having tracks extending tangentially and in particular to an apparatus for reading an optical dish which can accurately and stably control a jump-like movement of a light spot.
2. Description of the Background of the Present Invention
An optical disk comprises a spiral track or concentric tracks in which information is recorded in optically. During the reading or recording process, at least one light spot is produced by a light source such as a semiconductor laser device which scans a desired track or track portion of the optical disk. Whan another track or track portion is to be scanned, the light spot is moved rapidly from one track or track portion to the other track or track portion in a radial direction by a driving device which includes a mirror. Such a jump-like movement of a light spot in a radial direction is referred as "a track jump". In order to accurately control a track jump, a so-called tracking error signal has been used.
An example of a system in which a tracking error signal is used is described in U.S. Pat. No. 4,057,832. In the system, as shown in FIG. 6, the reading/recording light spot A is sandwiched between two light spots B.sub.1 and B.sub.2 which are used for tracking servocontrol. Reflected beams of the two light spots B.sub.1 and B.sub.2 are detected by a suitable detector (not shown), and the difference between the two detected values is obtained as a tracking error signal 31 which is plotted in the graph of FIG. 3(a), having time as abscissa and level of the tracking signal as ordinate. In the graph, the tracking error signal 31 indicates a change of the intensity level between the two reflected beams of light spots B.sub.1 and B.sub.2 when the light spots A, B.sub.1 and B.sub.2 move in a radial direction with a constant velocity. When the light spot A is positioned at the center portion "a" of a track Q.sub.1, the center portion "c" of an adjacent track Q.sub.2, or the intermediate portion "b" between the two tracks, the level of the tracking error signal 1 is zero. From the tracking error signal 31 shown in FIG. 3(a), a track jump signal 32 (FIG. 3(b)) is produced. The track jump signal 32 is applied to a driving device for positioning the light spots, so that the driving device is accelerated or decelerated in accordance with the polarity of the track jump signal 32 when accomplishing the track jump. When the polarity of the tracking error signal 31 is positive, the track jump signal 32 functions as an acceleration signal, and when the polarity is negative, the track jump signal 32 functions as a deceleration signal. The deceleration signal is applied for the same period of time as the acceleration signal.
However, acceleration characteristics and deceleration characteristics are not always identical with each other in all kinds of driving devices usually employed in such systems. When such systems are used for conducting high-speed track jumps in order to reduce the period of time required for track jumps, the response of the driving device to the track jump signal 32 is delayed. Hence, a track jump cannot be conducted stably and accurately in such a conventional system wherein the polarity of the track jump signal 32 is switched at the instant when the level of the tracking error signal 1 becomes to zero (i.e., when the light spot A is positioned at the intermediate portion "b"). Such a location corresponding to the light spot A when the polarity of the track jump signal is switched is referred to as "a switch position".
In order to solve the above-mentioned problem of the prior art, the present inventions performs a track jump as follows: When a tracking error signal 43 (FIG. 4(a)) decreases to a predetermined level R, i.e., when the light spot A moves to a location "d" which is positioned slightly short of the intermediate location "b", the polarity of a track jump signal 4 (FIG. 4(b)) is switched to negative so that the driving device is decelerated. The abovementioned problem may be solved by this system. As shown in FIG. 5(a), however, the peak value of a tracking signal is not uniform but varies in accordance with detecting conditions such as the focus-servo condition, the shape of the tracks, and the efficiency of the detecting of a reflected light beam. In FIG. 5(a), three examples of tracking signals 51a, 51b and 51c are shown. According to the present invention, the polarity of a track jump signal is changed when the level of a tracking signal decreases to a predetermined level R. When the tracking signals 51a, 51b and 51c are compared with the predetermined level R, therefore, the switch position corresponding to each of the track jump signals 52a, 52b and 52c moves as indicated by references ea, eb and ec in FIG. 5(b), respectively. Consequently, the application times ta, tb and tc of track jump signals 52a, 52b and 52c fluctuate, resulting in an unstable track jump.